Head-mounted displays are widely used in gaming and training applications. Such head-mounted displays typically use electronically controlled displays mounted on a pair of glasses or a helmet with supporting structures such as ear, neck, or head pieces that are worn on a user's head. Displays are built into the glasses together with suitable optics to present electronic imagery to a user's eyes.
Most head-mounted displays provide an immersive effect in which scenes from the real world are obscured and the user can see, or is intended to see, only the imagery presented by the displays. In the present application, immersive displays are considered to be those displays that are intended to obscure a user's view of the real world to present information to the user from the display. Immersive displays can include cameras to capture images of the scene in front of the user so that this image information can be combined with other images to provide a combined image of the scene where portions of the scene image have been replaced to create a virtual image of the scene. In such an arrangement, the display area is opaque. Such displays are commercially available, for example from Vuzix.
Alternatively, some head-mounted displays provide a see-through display for an augmented reality view in which real-world scenes are visible to a user but additional image information is overlaid on the real-world scenes. Such an augmented reality view is provided by helmet mounted displays found in military applications and by heads-up displays (HUDs) in the windshields of automobiles. In this case, the display area is transparent. FIG. 1 shows a typical prior-art head-mounted display that is a see-through display 10 in a glasses format. The head-mounted display 10 includes: ear pieces 14 to locate the device on the user's head; lens areas 12 that have variable occlusion members 7; microprojectors 8 and control electronics 9 to provide images to at least the variable occlusion members 7.
U.S. Pat. No. 6,829,095 describes a device with a see-through display 10 or augmented reality display in a glasses format where image information is presented within the lens areas 12 of the glasses. The lens areas 12 of the glasses in this patent include waveguides to carry the image information to be displayed from an image source, with a built-in array of partially reflective surfaces to reflect the information out of the waveguide in the direction of the user's eyes. FIG. 2A shows a cross-section of a lens area 12 including: a waveguide 13; partial reflectors 3 along with; a microprojector 8 to supply a digital image; light rays 4 passing from the microprojector 8, through the waveguide 13, partially reflecting off the partial reflectors 3 and continuing on to the user's eye 2. As seen in FIG. 2A, light rays 5 from the ambient environment pass through the waveguide 13 and partial reflectors 3 as well as the transparent surrounding area of the lens area 12 to combine with the light 4 from the microprojector 8 and continue on to the user's eye 2 to form a combined image. The combined image in the area of the partial reflectors 3 is extra bright because light is received by the user's eye 2 from both the microprojector 8 and light rays 5 from the ambient environment. FIG. 4 shows an illustration of a combined image as seen by a user from a see-through display 10 as described in U.S. Pat. No. 6,829,095 wherein the central image is an overly bright image composed of both an image of the ambient environment and a digital image presented by a microprojector. A reflectance of 20% to 33% is suggested in U.S. Pat. No. 6,829,095 for the partial reflectors 3 to provide a suitable brightness of the image information when combined with the image of the scene as seen in the see-through display. Because the array of partial reflectors 3 is built into the waveguide 13 and the glasses lens areas 12, the reflectance of the partial reflectors 3 should be selected during manufacturing and is not adjustable. Combined images produced with this method are of a low image quality that is difficult to interpret as shown in FIG. 4.
United States Patent Application 2007/0237491 presents a head-mounted display that can be changed between an opaque mode where image information is presented and a see-through mode where the image information is not presented and the display is transparent. This mode change is accomplished by a manual switch that is operated by the user's hand or a face muscle motion. This head-mounted display is either opaque or fully transparent. Motion sickness or simulator sickness is a known problem for immersive displays because the user cannot see the environment well. As a result, motion on the part of a user, for example head motion, does not correspond to motion on the part of the display or imagery presented to the user by the display. This is particularly true for displayed video sequences that incorporate images of moving scenes that do not correspond to a user's physical motion. U.S. Pat. No. 6,4976,49 discloses a method for reducing motion sickness produced by head movements when viewing a head-mounted immersive display. The patent describes the presentation of a texture field surrounding the displayed image information, wherein the texture field is moved in response to head movements of the user. This patent is directed at immersive displays.
Motion sickness is less of an issue for augmented reality displays since the user can see the environment better, however, the imaging experience is not suitable for viewing high quality images such as movies with a see-through display due to competing image information from the external scene and a resulting degradation in contrast and general image quality. Aspects of the problem of motion sickness associated with helmet mounted see-through displays is described in the paper “Assessing simulator sickness in a see-through HMD: effects of time delay, time on task and task complexity” by W. T. Nelson, R. S. Bolia, M. M. Roe and R. M. Morley; Image 2000 Conf, Proceedings, Scottsdale, Ariz., July 2000. In this paper, the specific problem of image movement lagging behind the head movement of the user is investigated as a cause of motion sickness.
U.S. Pat. No. 7,710,655 describes a variable occlusion member that is attached to the see-through display as a layer in the area that image information is presented by the display. The layer of the variable occlusion member is used to limit the ambient light that passes through the see-through display from the external environment. The variable occlusion layer is adjusted from dark to light in response to the brightness of the ambient environment to maintain desirable viewing conditions. FIG. 1 shows a variable occlusion member 7 located in the center of the lens area 12 wherein the variable occlusion member 7 is in a transparent state. FIG. 2A shows a variable occlusion member 7 wherein, the variable occlusion member 7 is in a darkened state. Similarly, FIG. 2A shows a cross-section of a variable occlusion member 7 in relation to the waveguide 13 and the partial reflectors 3 wherein the variable occlusion member 7 is in a transparent state. FIG. 2B shows the cross-section wherein the variable occlusion member 7 is in a darkened state so that light rays 5 from the ambient environment are substantially blocked in the area of the variable occlusion member 7 and light rays 5 from the ambient environment only pass through the transparent surrounding area of lens area 12 to continue on the user's eye 2. As a result, the combined image seen by the user is not overly bright in the area of the variable occlusion member 7 because substantially only light from the microprojector is seen in that area. FIG. 3 illustrates the variable occlusion member 7 in a dark state. FIG. 5 shows an illustration of the combined image as seen by the user where the variable occlusion member is in a darkened state, as in FIG. 3. Although image quality is improved by the method of U.S. Pat. No. 7,710,655, compensating for head movement of the user to provide further improved image quality and enhanced viewing comfort is not considered.
There is a need, therefore, for an improved head-mounted display that enables viewing of high quality image information with reduced motion sickness and improved viewing comfort for the user.